Approximately 50 million subscribers access the Public Switched Telephone Network (PSTN) via DLC systems in North America. These subscribers are now demanding that their service providers support Digital Subscriber Line (DSL) services to enable the subscribers to obtain high-speed Internet access and data services. Although the service providers, typically Local Exchange Carriers (LECs), desire to offer the DSL services to their subscribers, the LECs face several problems with deploying the equipment required to support DSL services in existing DLC systems. For example, in the absence of remote access equipment, the LEC can not reliably provide DSL services to a subscriber that is located more than a few thousand feet from the CO. For subscribers in remote locations that are serviced via remote access equipment, such as a DLC system, the LEC may find that the existing DLC system lacks the physical infrastructure to support the addition of a sufficient number DSL channels to satisfy subscriber demand.
The LEC may elect to use a rack-mounted DSL Access Multiplexer (DSLAM) within the DLC field cabinet to support the distribution of DSL services in locations remote from the CO. The LEC may install the DSLAM at the DLC if physical space is available within the DLC field cabinet to accommodate installation of that rack-mountable device and if subscriber demand for DSL services is sufficient to warrant the purchase and installation of the DSLAM. The DSLAM manages the distribution of data in the downstream path, toward the subscriber, and in the upstream path, toward the data network. In the downstream path, on a packet-by-packet basis, the DSLAM determines which subscriber or DSL channel each data packet is intended for and routes it to the appropriate subscriber. In the upstream path, the DSLAM collects the data packets from each of the DSL channels, buffers the data packets, and transmits the data packets toward the data network, such as a network supporting the ATM protocol.
FIG. 1A is a block diagram illustrating the architecture for a DSLAM configured for installation within an available rack within a DLC field cabinet. As shown in FIG. 1A, a DSLAM is typically implemented as an assembly of circuit boards connected to a common backplane in an equipment bay of the DLC system. For a representative DSLAM 100, the circuit boards typically include a common ATM controller 105 and multiple DSL port boards 110a–110d. Each DSL port board 110a–110d typically includes multiple DSL port circuits or lines. A backplane provides a common bus structure that communicates the Input/Output (I/O) and ATM signals between the circuit cards. Specifically, I/O and ATM signals are communicated between the backplane 115 and the ATM Controller 105 and the DSL port boards 110a–110d. The backplane 115 typically includes circuit board receptacles that accept the insertion of the ATM controller 105 and the DSL port boards 110a–110d. 
For many DLC systems installed by LECs prior to the advent of DSL services, the DLC field cabinet fails to contain a DSLAM or sufficient cabinet space to accommodate the installation of a rack-mounted DSLAM. Moreover, the LEC is likely to find it costly to install a DSLAM within the DLC field cabinet if that DSLAM services only a small number of subscribers in a location remote from the CO. To address these concerns, LECs may elect to install smaller DSLAM-like multiplexers, referred to as mini-Remote Access Multiplexers or mini-RAMs, within the DLC field cabinet. A mini-RAM is typically designed to serve a relatively small number of DSL subscribers and includes a physically small form-factor suitable for installation in a field cabinet. Similar to the design of a representative DSLAM, a mini-RAM typically includes an ATM controller and one or more DSL port cards, each connected to a common backplane for the distribution of ATM and I/O signals.
Although DSLAMs and mini-RAMS represent potential solutions to the distribution of DSL services to remote subscribers, DLC field cabinets represent a complex environment in which to install such additional equipment. Field cabinets are often completely or nearly full of rack-mounted devices, thereby leading the LEC to conclude that it is not possible or at least not advisable to install additional equipment at the DLC. Moreover, the installation of new equipment within a DLC field cabinet often results in a complicated rewiring of the electrical power bus.
In view of the foregoing, there is a need to provide telecommunication services, such as ATM, Multiple Protocol Label Switching (MPLS) or Frame Relay services, to remote subscribers serviced by a DLC or CO without installing additional large form-factor equipment within a field cabinet or requiring a complicated rewiring of telephony equipment. There is a further need to extend such telecommunication services capability to the installed base of older DLC systems and to expand the delivery of advanced data by both existing and newer DLCs. The present invention solves these needs by providing an expandable capability to deliver telecommunications services with a master unit and one or more expansion units, each coupled to a flexible expansion link, for installation within the existing space of a DLC field cabinet or a CO.